For those people hating on the age gap between Lee Jinwook and Jung Chaeyeon, I'm not sure if they've seen some of Hollywood's most successful onscreen couples whose age gaps were the same or even much bigger than the 16 years between LJW and JCY.
Take, for example, the partnership of the great Humphrey Bogart and Lauren Bacall. In their first film together, he was 45 and she was 19, almost 20. They made her play the role of a street-smart and sassy bar singer and German resistance sympathizer opposite Bogart's anti-hero character. She pulled off her role so well despite her youth. Everyone saw how their onscreen chemistry sparked and sizzled and the film became a success. They did more films together after this. (Incidentally, they fell in love for real, got married and had two children together before Bogart passed away).
Or the one of Cary Grant and Grace Kelly in the classic Hitchcock film "To Catch A Thief" as the notorious burglar, John Robie, and his potential victim, American heiress, Frances Stevens. He was 51 at the time to her 26. The film was a success. No one really thought Grace Kelly was too young to play Frances opposite Cary Grant.
Like these famous Hollywood actors, Lee Jinwook and Jung Chaeyeon are actors too. It's their job to play roles that may be close to their age or older than they really are. It just so happens that they have a similar natural and intense chemistry that these Hollywood legends had.
Chemistry is not necessarily tied to the age of the actors. Sometimes, despite the significant gap, they're still able to establish a connection that makes the partnership work. You can't fake that or force that. It just comes.
That's why some dramas don't work despite the great storyline, while others become highly successful in spite of them being tropey and mediocre.
When we get lucky, we get both the great storyline and the leads that light up the screen with their chemistry. That's when the drama or film becomes unforgettable.